Internal combustion engines (ICE) are often called upon to generate considerable levels of power for prolonged periods of time on a dependable basis. Many such ICE assemblies employ a boosting device, such as an exhaust gas turbine driven turbocharger, to compress the airflow before it enters the intake manifold of the engine in order to increase power and efficiency.
Specifically, a turbocharger is a centrifugal gas compressor that forces more air and, thus, more oxygen into the combustion chambers of the ICE than is otherwise achievable with ambient atmospheric pressure. The additional mass of oxygen-containing air that is forced into the ICE improves the engine's volumetric efficiency, allowing it to burn more fuel in a given cycle, and thereby produce more power.
A variable-geometry turbocharger (VGT) is a type of a turbocharger usually designed to allow the effective aspect ratio (A:R) of the turbocharger to be altered in line with engine speed and thus facilitate increased ICE operating efficiency. VGTs tend to be more common on compression ignition or diesel ICE's, as compared to spark ignition ICE's, because lower exhaust temperatures of diesel engines provide a less extreme environment for the movable components of the VGT.